Muscle provides a unique opportunity for quantifying a bioenergetic and energy balance system in living tissue. This study provides a special opportunity in this Resource to evaluate the mechanisms underlying intracellular chemical fluxes using a parallel approach combining experimental and modeling studies. The muscle system can be characterized as follows. Muscle activation leads to increase of cellular ATPases by more than 10-fold compared to the resting state. This increased ATP breakdown is balanced by ATP synthesis from the metabolic products of contraction as well as the activation signal (i.e., Ca++). These signals activate mitochondrial oxidative phosphorylation, which is the predominant process (>95%) driving the ATP resynthesis needed to attain energy balance. Our measurements of the relevant intracellular metabolites provide quantitative tests of how these sub-systems are organized and integrated to achieve energy balance. This project will develop quantitative models based on known enzyme and organelle properties and mechanisms. Our experimental systems provide tests of the kinetics and magnitudes of the chemical changes and fluxes predicted by these models. The result will be a comprehensive model of intracellular energetics applicable to striated muscle in general.